The present invention generally relates to a developing apparatus for an electrophotographic copying apparatus or the like, and more particularly, to a toner replenishing arrangement for a developing apparatus for use in an electrophotographic copying apparatus, etc.
Commonly, in an electrophotographic copying apparatus, an image of an original document is formed on the surface of a photosensitive member as an electrostatic latent image, onto which toner is caused to adhere so as to form a visible toner image. Then, this toner image is transferred and fixed onto a copy paper, and thus, the image of the original document is reproduced on the copy paper.
In the above processing, since the toner is consumed for each copying, it is necessary to replenish the developing apparatus with toner by a replenishing means provided within the copying apparatus main body, as well as to detect that the toner for the replenishment in the replenishing means has run out to be empty.
As one practice for replenishment of toner, there has been known a constant amount replenishing method. In this practice, a predetermined amount of toner is replenished per each one copy cycle.
Conventionally, since an average toner consumption for one ordinary copy paper sheet may be found by the copy paper size most frequently adopted and an aveage image density during ordinary copying, it has been a usual practice to set the toner replenishing amount equal to such average toner consumption, whereby owing to the fact that the toner consumption becomes equal to the toner replenishing amount in a long term, the toner concentration within the developing aparatus may be stabilized.
However, one of the disadvantages of the constant amount replenishing methods as referred to above is that it is impossible to cope with the variation of toner consumption for each copy paper size. In other words, even with respect to the same image density, if the copy paper sheet size is increased, the toner consumption is also increased by that extent, while on the contrary, upon reduction of the copy paper sheet size, the toner consumption is also decreased. Thus, if a large amount of copying is effected by copy paper sheets in large size or small size in a state not on the average, the state in which the balancing between the replenishing amount and the consumption is largely lost continues for a long period, thus resulting in a deviation of the toner concentration within the developing apparatus to a large extent.
In order to solve the problem as described above, there has also been proposed another method in which, by preliminarily setting the toner replenishing amount equal to the toner consumption in the case where copy is taken at the average image density for each size of the copy paper sheets, by copy paper sheet size is detected during the copying so as to replenish toner in the amount corresponding to said size. By the above practice, the problem related to the deviation of toner due to variation of the copy paper sheet size has been solved.
Another disadvantage inherent in the constant amount replenishing method is that said method can not cope with the variation of the toner consumption following the variation of the image density. In other words, deviation of the toner concentration within the developing apparatus occurs when a large amount of copying is continuously taken in different image densities with respect to the average image density referred to earlier.
For solving the above problem, there has been conventionally proposed a method in which, with a reference toner image formed on the surface of a photosensitive member, the toner replenishing amount is varied so as to correspond to the toner concentration within the developing apparatus as detected through detection of the density of said toner image, for example, in Japanese Patent Laid-Open Application Tokkaisho No. 58-190966.
In the above practice employing the reference toner image, there is a possibility that an erronous toner replenishment is effected of a sensor for detecting the reference toner image becomes faulty, and therefore, it is required that the sensor is arranged to be checked for any fault at all times.
Meanwhile, when the toner to be replenished has run out in the toner replenishing means, the toner replenishment can not be effected, and therefore, in the conventional arrangements, an exclusive sensor has been provided for the detection of the state for "toner empty".